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William R Bobier, Vidhyapriya Sreenivasan, Elizabeth L Irving; Can current models of accommodation and vergence predict accommodative behavior in myopic children?. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2729.
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Models predict that adaptive tonic responses (Aadapt and Vadapt) are proportional to the strength of the initial reflexive responses of accommodation (acc.) and vergence. Tonic increases in turn attenuate their respective cross link outputs of AC/A and CA/C (1). We investigated whether such predictions resulted when testing myopic children.
Reflexive, adaptive and cross link components of acc., and vergence were measured in response to incongruent stimuli (-2D, +2D near adds and 10pd base out (BO) prisms). Participants were 28 myopic and 25 “emmetropic”(emms) children aged 7 to 15 years. Myopia ranged from -0.75 to -6 D and between +0.5 and +1.5 D for emms. Subjects were divided into near phoria groups exos, orthos and esos.. Twenty-five myopes and emms completed the add study and of that group 22 myopes and 17 emms completed the prism study .Children sustained fixation through adds (at 33cm) or 10 BO prism (at 4M) for 20 min. Acc. responses (PowerRefractor) and phoria adaptation were measured at frequent intervals.
Myopes responded with greater lags of acc. compared to emms (P=0.01) but acc. lag varied as a function of phoria-group (under binocular viewing) such that exophores displayed significantly less lags of acc. compared to esophoric children (p=0.020). The response AC/A was higher in esos vs exos in both groups (p= 0.04) and higher in myopes (p=0.005). However, the CA/C measures were not significantly different for either phoria or refractive group. Myopes showed higher Aadapt through -2D adds vs emms (p=0.01). Esos showed higher Aadapt in both groups (p=0.01). Vergence adaptation was significantly correlated with the degree of fusional vergence induced by the AC/A (p<0.05) crosslink and the BO prism.Myopes showed significantly reduced vergence adaptation when viewing through 10BO prism and +2D but not -2D adds.
Current models do not predict (i) the combination of high levels of AC/A; Aadapt and acc. lag found in myopic children nor(ii) the high Adapt in esophoria found in both groups(2). Vadapt and CA/C behavior are consistent with adult models.CA/C responses show normal accommodative plant behavior, suggesting a neural etiology for these accommodative differences. 1. Schor CM. O.V.S.. 2009,86: 1788-1802: 2. Schor C, Horner D. O.P.O 1989,9:264-8.
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